The betatron is a charged particle accelerator which uses for its accelerating mechanism the electric field produced by a changing magnetic flux, linking the orbits of the electrons or other charged particles. The changing accelerating flux is generally produced by a coil or coils, energized with alternating current. The electrons or other charged particles are constrained to follow a generally circular orbit by the magnetic guide field at the orbit. Such guide field provides the centrally directed force which produces the circular motion. The guide field increases with increasing electron energy so that the successive orbits are of essentially constant radius. The magnetic guide field is also generally an alternating current field.
The changing magnetic field, linking with the orbits, serves to accelerate the electrons in the orbital beam. Simultaneously, the magnetic guide field at the orbit changes so as to maintain a constant radius while the energy of the orbiting electrons is increasing. While the flux linking with the orbits is increasing, an electromotive force is developed around the orbital path so that a tangential force is applied to the electrons or other charged particles, thus accelerating them to high energy. One may think of an ordinary transformer as operating in much the same manner, with the orbiting electrons replacing the secondary winding, except that the electrons in the secondary winding suffer so many atomic collisions that they acquire only a slow average velocity.
This constant radius characteristic of betatron accelerators is a consequence of the 2-to-1 rule which states that the rate of change of the space-averaged flux density in the central iron core within the orbit must equal twice the rate of change of the guide field at the orbit. Both fields must increase proportionately with time, the central field to produce the acceleration, and the guide field at the orbit to retain the particles of increasing energy in an orbit of constant radius.
A betatron accelerator employs means for injecting electrons or other charged particles into the accelerating tube, with a timing and an energy to produce acceleration of the electrons in substantially circular orbits around the accelerating tube. The electrons may be injected by an electron gun, comprising a hot filament and a grid, mounted close to the wall of the accelerating tube. The grid is at a positive potential to inject the electrons with a suitable initial velocity. Injection occurs when the guide field is slightly above zero, at a value consistent with the injection energy.
The alternating current guide field may be biased by a superimposed direct current field, to keep the guide field positive throughout all or most of the alternating current guide field. With the biased guide field, acceleration takes place during a greater portion of the alternating current cycle, approaching a half cycle of the alternating current which drives the magnet. The final energy is reached at the peak of the field, at which time the orbiting electrons are extracted so that they strike a target.
Betatrons are usually operated at 50, 60 or 180 cycles per second and are often resonated with a large capacitance. The entire magnet structure is usually made of laminated iron sheets to minimize eddy currents. The accelerating tube is in the form of an annular vacuum tube or chamber which surrounds the central core. In many cases, the central core has an air gap, which serves the function of controlling the reluctance of this part of the magnetic circuit so that the 2-to-1 rule will be observed. The vacuum tube may be made of glass, quartz or ceramic and may be lightly silvered or otherwise metalized on the inside surface, to prevent the accumulation of static charges from misdirected electrons. Such static charges would tend to interfere with the maintenance of circular orbits.
As the final energy of the orbiting electrons is approached, the orbits are expanded or contracted by various means, so that the electrons strike a target on the outer or inner wall. For example, the orbits may be expanded or contracted by altering the flux in the central core. The change in flux may be accomplished by placing in the core's air gap one or more pieces of magnetic material which saturate at the desired flux density. Another method of extracting the electrons is to pulse currents through special windings at the proper moment. In this way, the electron beam, which is of small cross section, is swept onto the target at a rate depending on the abruptness of the flux perturbation.
The final energy of the accelerated electrons can be approximately doubled, for a betatron of any particular radius, by utilizing biasing techniques. One such technique produces a field-biased betatron, in which a steady positive biasing field at the orbit is superimposed upon the usual alternating guide field, so that the net guide field changes from zero, or slightly more, to its positive peak value during a half cycle, while the purely alternating accelerating flux in the core changes from its negative peak to its positive peak. An alternative biasing technique is called flux-biasing, which is a pulsed technique in which the central core is given a permanent negative bias and is pulsed to an approximately equally large positive value during the time in which the guide field is driven from zero to its peak. It is also possible to employ a combination of flux-biasing and field-biasing.
A number of circuits have been devised, following these general lines, with auxiliary windings around various portions of the magnetic yoke and poles of the betatron accelerator. In some instances, the air gap in the core is eliminated so that the stored energy is reduced, while also reducing the cost and complexity of the power supply. In these cases, some other method of maintaining the 2-to-1 ratio is needed, such as the use of auxiliary guide field coils. In general, the use of biasing techniques reduces the weight of the iron magnetic circuit for a given energy.
In the conventional betatron accelerator, there is a single annular accelerating tube, in which electrons or other charged particles are accelerated around orbits in one direction of rotation, during the portion of the alternating current cycle in which the accelerating flux is changing in one direction, as between its negative peak and its positive peak. A full wave betatron has also been proposed, in which a second electron beam is injected into the betatron tube in the opposite direction of rotation, during the remainder of the alternating current cycle, when the flux is returning from its positive peak to its negative peak. However, the full wave betatron does not utilize the advantage of direct current field biasing. The present invention deals with the problem of providing a betatron utilizing two electron beams, while also obtaining the advantage of providing direct current biasing.